The present invention relates to a spectacle glass with an Abbe-number greater than 45 and a refractive index greater than 1.56, particularly distinguished by its very low density.
Current commercially available spectacle glass which is colorless in the visible spectrum range, has a refractive index n.sub.d of approximately 1.523, an Abbe-number of about 56 and a density of at least 2.55 gcm.sup.-3. Plastic material which is also in common use for spectacle lenses has a refractive index of 1.500, an Abbe-number of 58 and a density of 1.32 gcm.sup.-3. The demand for an inorganic spectacle glass with a higher refractive index as compared with the currently common refractive index of 1.523 has been somewhat satisfied by the glasses of DE-PS 22 59 183.
Such inorganic glasses, particularly as compared with the spectacle glass with n.sub.d of 1.523 and even more so in comparison with the plastic material, have the great advantage of permitting, for the same diopter in a corrective prescription, significantly slimmer peripheral thicknesses in negative lenses and significantly thinner central thicknesses in positive lenses. They are therefore more attractive to the spectacle-wearer from the important aspect of cosmetic appearance. The density increase which is normally proportional to refractive-index increase can be relatively restricted but in most cases it is not possible to go below a density of 3.0 gcm.sup.-3.
These glasses are particularly suitable for lens prescriptions in the higher positive or negative diopter ranges, normally over .+-.2.5 dpt.
On the other hand, the majority of correction prescriptions fall within the range between +2.5 dpt and -2.5 dpt. Within this range, the two curves of a spectacle lens (the outside curve and the inside curve facing the eye) are not excessively dissimilar so that the higher refractive indices cannot have their beneficial effect while the relatively higher density of inorganic lenses is felt to be a distinct disadvantage by comparison with plastic lenses.